In order to enable the efficient dewatering of organic sewage sludge by mechanical means such as filtration or centrifugation, it is the usual practice to add floculating agents to the sludge prior to dewatering. The most commonly accepted conditioning method is to mix ferric chloride with the sludge and thereafter to mix virgin or slaked lime to the sludge mixture. The amount of ferric chloride which is generally used lowers the pH of the sludge to about 3.0 or less and the subsequent addition of lime raises the pH to a value in the range of about 10.5 to 11.5. While this well known process permits the dewatering of the sludge in a reasonably short time, it has several disadvantages.
One of the more important disadvantages of this widely used conditioning process is the fact that the resulting dewatered sludge has little if any commercial value and must be handled as a waste product. Moreover, the conditioning agents themselves when added to the sludge increase the weight of the dry solids in the sludge by a factor of about twenty percent. This added weight and bulk in turn increases the disposal cost. In addition, however, since the sludge and the removed solids have a very high alkalinity the cost of disposing of these solids is disproportionately greater. Also, the high alkalinity of the conditioned sludge necessitates frequent cleaning and descaling of the mechanical dewatering equipment used in the overall process.
Published studies describe test work involving conditioned sludges having pH values as low as 8.0 to 7.0 but there are no reports of studies conducted with sludges having lesser pH values. See for example, Tenny et al--"Chemical Conditioning of Biological Sludges for Vacuum Filtration" at 42R1-1970 of the Journal Water Pollution Control Federation. These studies indicate that filtration times of the conditioned sludges increase appreciably as the pH decreases below a pH of 7.